Wireless communication systems between base stations and mobile stations, such as mobile phone handsets or Personal Handyphone System (PHS) devices, have been in practical use. In such wireless communication systems, a plurality of base stations are provided to form a plurality of cells indicating communication regions. A given mobile station may be contained in one of these cells. The mobile station then performs a hand over from a base station in communication to another base station when moving among cells. Such wireless communication systems are susceptible to the influence of various issues, such as decreased signal levels due to shadowing and propagation loss, as well as increased interference from surrounding cells. In such wireless communication systems, decreased signal levels and increased interference may result in decreased reception (i.e., Signal to Interference plus Noise power Ratio, or SINR), particularly for mobile stations near the edges of cells.
One technique for improving reception involves site diversity, wherein the same signal is made to be transmitted from a plurality of adjacent base stations, with the received signals being combined at the receiving mobile station. Meanwhile, another technique for suppressing interference power involves using directional antennas at the base stations to subdivide cells into equal sectors at specific angles. By means of such techniques, it becomes possible to amplify the desired signal power in wireless communication systems, while also improving reception by suppressing the interference power from surrounding cells.
In addition, there exists technology for such wireless communication systems wherein, for example, the directional antennas at base stations are synchronously rotated, or wherein upper and lower antennas that complement each other's zones are disposed at base stations. In so doing, the region where high bitrate communication is possible (i.e., coverage) can be expanded.
However, in the technology of the related art described above, there is a problem in that the reception is only improved by a small amount. More specifically, in wireless communication systems in accordance with the related art, site diversity or sector formation using directional antennas at the base stations is conducted for mobile stations positioned at the edges of cells, as described above. However, in wireless communication systems of the related art, the incoming power from surrounding cells and sectors is low from the start for mobile stations positioned at the edges of cells. As a result, in wireless communication systems of the related art, the improvement in reception becomes small for mobile stations positioned at the edges of cells where the incoming power is low, even if site diversity is conducted.